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How to Start a Jet Engine
I know how the basics of how jet engines work. I know that air goes into the compressor as the first step of the journey. However, where does the air come from that goes into the jet engine at such high speeds it turns the rotors. I don't think its coming from the air because, the plane wouldn't be moving and no air would be flowing at a high rate. Also, once the jet engine is started up, how does air keep coming into the jet engine in flight. From air in the sky?
Is there a difference in the processes for fighter jets and regular commercial planes?

https://i.ytimg.com/vi/NYq0nf9fSjY/maxresdefault.jpg :D

In all seriousness, nice question!

Very related: how-are-turbine-engines-started

Inside some jet engines there is a powerful electric motor that starts the engine by spinning the compressor up to speed, at which point the fuel flow and the ignition spark are turned on, and the resulting combustion then forces the turbine to spin. The turbine's power output is sent to the compressor stage to spin it, and the electric start motor can then be turned off.
On other jet engines, a small gas turbine is used to blow hot combustion exhaust through the main turbine, which spins the main compressor up to speed, then the fuel and spark come on, etc. The start turbine is then shut down once the main engine starts.
Note that whenever the engine is running, the compressor stage at the entrance to the engine is actively sucking in tremendous quantities of outside air and jamming that air into the combustors in the center of the engine, where it is burned with fuel. The power of the exhaust from the combustors then spins the turbine at the outlet end of the engine, and that power is fed back to the compressor on a spinning shaft to keep it sucking in more air.
Some military jet engines can be started by connecting a huge blower to the engine inlet and forcing compressed air through the engine, to spin the turbine and get the compressor going. This is called an air start. Some military jets can also be started by burning a solid-fuel cartridge made of black powder and shooting the hot exhaust into the turbine stage of the engine to spin it, and thereby spin the compressor stage up to speed and get things going.
Other military jets are started by a start cart which contains a powerful gasoline engine that spins a shaft geared onto the main shaft of the jet engine. Once the jet is up and running, the start shaft is disconnected from the jet and the start cart is rolled out of the way.

High pressure air is used to turn an air starter inside the engine. That starter is connected to the inner shaft of the engine via gearbox.
The source of the high pressure air can be several things:

The auxiliary power unit (APU), a small gas-turbine engine in the tail of the airplane which itself is started with an electric motor
A ground air starter, a compressor either on a small vehicle or directly connected to the air-bridge
Another engine that is already started via cross-bleed.

Smaller jet-engines are also started with an electric motor connected to the inner shaft with a gear box, similar to an air starter. The electric starter is often used as a generator once the engine is running. This makes this concept very light and efficient, few moving parts, no high temperature/high pressure air ducts that can leak and cause issues, etc.

I know that air goes into the compressor as the first step of the journey

From the way you mention this phrase it sounds like you have a slight misunderstanding of how a jet engine works.
As your confusion illustrates. Air does not just "flow" unless there is a pressure difference. A pressure difference can be created by several means: atmospheric heating and cooling (which is how we get wind), airfoil moving through air (fans), impellers accelerating air around a circular path (blowers) etc.
Air does not go into the compressor. The compressor pushes air through the engine.
In a modern jet engine the compressor is designed like a fan. Thus the action that causes the air to flow through the compressor is the same as a table or ceiling fan: the airfoil of the compressor blade moving through air. Just like a regular fan you find in most homes, this causes the airfoil to create a high pressure region behind it causing the air to be blown backwards and a low pressure region in front of it causing air to be sucked into the engine.
In older jet engines from the 1940s and 1950s the compressor works kind of like a leaf blower or a car turbocharger compressor with impellers that push air sideways due to centrifugal force. Again, this causes a high pressure zone behind the impeller and the exit of the air inside the impeller causes a low pressure zone in front of it sucking air inside the compressor.
As in any application using fans, blowers or compressors such as a table fan or hair dryer or a turbocharger it is the motion of the compressor that causes air to move in a jet engine.
But unlike a table fan or hair dryer it is the hot jet exhaust that powers the compressor which is the same as how turbochargers work in cars. In fact both aircraft jet engines and car turbochargers are called the same thing: turbines.
How a jet engine is started depends on the design of the engine. Some use an electric motor, some use hydraulics, some use a piston engine and some use a separate "starter" blower to initially blow air into the engine to start the compressor spinning. There are even jet engines designed to start spinning the compressor by firing a shotgun shell inside the engine: these are called catridge start (google "catridge start jet engine" to find interesting youtube videos). You can read more about the various ways to start a jet engine (technically called "gas turbines") on wikipedia: https://en.wikipedia.org/wiki/Aircraft_engine_starting#Gas_turbine_engines


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Is airflow deflected inwards or outwards by wing sweep?
We are often told that wing sweep causes the airflow to be deflected outwards. I can understand this for the underside, but for the upper surface it seems inconsistent with other effects.
For example on a sharply-swept delta, sideways flow out from below, up around the leading edge and back in again over the upper surface becomes dominant. It feeds the vortices employed by Concorde to enhance lift at low speeds. The sharper the sweep, the more the air over the upper wing flows inwards. The same effect is seen in miniature with tip vortices on a straight wing. This makes sense to me because the pressure above the wing is low, so that would naturally attract air in towards that zone.
So, does sweeping a straight wing really reverse such effects, or do say wing fences on the upper surface actually stop the air flowing inwards?

Thanks,  certainly helps: it shows that both can happen in different places.

Note that first you are asking about swept wings then you change subject to deltas. The filled-in area of delta wings may seem insignificant but they cause the wings to behave very differently from swept wings. Some of the drastically different characteristics of deltas over swept wings: deltas can employ the creation and detachment of vortices to create lift (vortex shedding) an effect not seen on swept wings, deltas generate high drag when turning causing it to slow down but also allow it to achieve tighter turning circle, deltas surprisingly retain aileron control very deep into stall etc.

@slebetman Yes one has to be careful. But in other ways the delta is often treated as a class of swept wing and much theory applies to both: certainly, both feature wing sweep in some form. So I think that one helpful strand of these answers might be to bring out any relevant aspects where the delta acts as a typical swept wing and where it differs.

The primary difference is the filled-in area creates a low pressure zone (because air could not flow into it from below) that doesn't exist in swept wings. You can easily imagine such a low pressure zone may suck airflow inwards instead of outwards. I'm not sure if it does. What I do know is that it encourages the formation of vortices along the entire leading edge (instead of the usual tip vortex) and also encourage those vortices to detach and join the airflow (leading edge vortex shedding). As such the difference feels quite relevant to me

@slebetman That may be so at subsonic speeds, but if the local flow over the aft section is above Mach 1 then the flow over the fore section cannot be affected by the geometry aft of the shock wave and the difference disappears.

As a partial answer following up on those to date, here is an image from Where is the spanwise flow? How does the span wise flow point the air towards the wingtip?, showing a snaking in-then-out flow over the wing. Note the inward flow over the leading edge and wing section, which is just the point where wing fences are located.

On sharply-swept deltas which sit inside the bow shock cone at supersonic speeds, 1980s studies of supercritical attached flows show the air flowing upwards round the leading edge and forming a crossflow to meet an inboard "crossflow shock". See for example W.H. Mason, “A Wing Concept for Supersonic Maneuvering,” NASA CR 3763, December 1983. But it is less clear to me whether this inbound crossflow is physical, or a theoretical notion along the wing profile which, due to the sharply tapering chord with span, physically might still flow outwards:

This contrasts with the low-speed vortex flow over such wings (e.g. Concorde) at high angles of attack, where the vortex inflow is detached and the boundary layer flows outwards.
So it seems the full the answer must be complicated, with the pressure distribution along the chord, and hence the in-out flow changes, depending on the angle of sweep, the speed of flight, the angle of attack and how far above the wing you are considering.

Yes, it is indeed complicated. You will even see a change in flow direction over the thickness of the boundary layer. Near the wall and past the suction peak the flow is outwards while the outer flow of the boundary layer still flows inwards, as does the outer flow.

And regarding fences: They sit at an angle to the local flow which produces a vortex. This vortex, not the fence itself, helps to prevent the outward movement of the boundary layer. Complicated indeed.

@PeterKämpf So even a fence locally experiencing inward flow would create a vortex impeding the outward flow further back?

@PeterKämpf so, "bidirectional" like the Chicago River!

It is deflected outwards by swept back wings and inward by front sweept wings. That is  the main job for wing fence (to prevent the outside flowing of the airflow).

That is the received wisdom which I am specifically questioning here.

First portion of contact the airflow go inward is true but then it travels outward. All depends on the AOA and speed at the moment. Some spelling along with the Leading Edge all the way from the root of the wing to the tip if the Leading Edge it has big  curvature  and not sharp enough

But most of the air travelling on the top side of the that type of wings is to the tips

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Did the Mig 21 have differential stabilators to help the ailerons?
The MiG 21 had all-moving tailplanes or stabilators. When rolling, did one stabilator pitch up and the other down to act as secondary ailerons and help the main ones on the wing?

This video should at 6.35 gives a good view of a roll - no tailplane activity. https://www.youtube.com/watch?v=uePIzbYI7BU

Spoiler alert - no, the stabilisers didn't pitch when rolling in the video.

"Spoiler alert - no, the stabilisers didn't pitch when rolling in the video"-because is 1 G rolling maneuver.

No.
MiG-21 (any modification) didn't need it and didn't have it. With its low-aspect-ratio delta wing and most mass concentrated in the centre, it was agile enough with standard ailerons.
The elevator had a single hydraulic booster driving both halves of the stabiliser.
Differential stabiliser was used on MiG-21's successor, MiG-23. That one had no ailerons at all; it had full-span flaps on its swing wing instead. Roll was controlled by the spoilers and the differential stabiliser.

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Flying wing vs box wing aircraft design
I've been reading some articles online about flying wings aircraft design. But articles about box wing design (of similar depth to flying wing articles) seem harder to find.
For illustration, here are two example aircraft, one of each design, i.e. flying wing and box wing respectively.

Northrop YB-35

Sunny box wing aircraft
From the two images above, I can easily figure out a similarity and a difference between the two aircraft: The swept wing and the vertical stabilizer
What are the major similarities and differences between the two aircraft configurations? For example, if the sunny box wing aircraft doesn't have a swept upper wing, will it still be stable in flight, and is wing sweep necessary for flying wings and box wing aircraft?

How else am I supposed to present this question? I think we need a better version of StackExchange :(

Someone sees it as three separate questions rather than one. I will take the liberty of editing it a little and if you don't like it then one of us can undo my edit.

Sweeping the wing gives a little pitch stability, and the rear mounted props also helped the YB-35.  Lack of length makes it harder to control the wing aerodynamicly in pitch,  but hang gliders do this by shifting weight.  Both designs could have straight wings and tails, but the sweep also gives a bit more yaw stability.  One possibility for the boxwing would be wing mounted rudders and a rather bird-like horizontal tail.  But stability is key for safety, even if it does not look quite as "modern".

A good place to start is the biplane flying wing with endplates that box it in, so that it is both a flying wing and a box wing. The Dunne D.5 of 1910 was the first certified stable airplane ever to fly. It was a swept design, with pronounced wash-out towards the tips, effectively putting the tail at the end of the wing. But its ancestor the D.1 glider had proved stable without any endplates. Although they did improve the stability of the D.5 and D.8, their main function was to reduce tip losses. They even lacked a rudder; the planes turned OK using the ailerons alone but could not land in a crosswind.
By comparison the box wing just flips the lower wing backwards, so it acts as the horizontal stabilizer and there is no need for washout. But it also loses directional stability so a tail fin or similar is necessary. For this reason many box designs, such as the Sunny, sweep it less than the front, so that the swept fore wing and endplates are enough to stabilize it.
The monoplane swept flying wing typically has no endplates. Dunne achieved the same effect by sharply drooping the wing tips. But in most designs, the washout reduces lift at the tips enough to limit tip losses.
Sweep is not strictly necessary. If you use an airfoil with reflex camber, tilted up at the trailing edge such that it has a stationary centre of pressure as AoA changes, it will be inherently stable and no other tricks are needed. But it will be very sensitive in pitch and the controls will not be well balanced. Also, it will be unstable or minimally stable in yaw unless you play tricks similar to Dunne's, such as adding endplates and toeing them in to create differential drag.

With this answer of yours, I think I'll go with the swept wing design. But sir, why is it so difficult to ask questions appropriately on stack exchange? How else am I supposed to present this question to be worthy of been asked here?


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What kind of antique WWI aircraft model is this? Inventor's prototype?
I've discovered an interesting WWI era model of an aircraft and I am trying to find out what could it be. It is 29" long and it looks like not a hobby model, but an inventor's prototype. I love the idea of the extra fuel tank above the cockpit. It connects with a bigger fuel tank near the engine. Any ideas?

Welcome to the site. Personally, I am not sure that above the cockpit there is a fuel tank, looks like an attachment for the top wing

The unusual shape is similar to a Curtiss Carrier Pigeon.

Hi Federico! There is a fuel tank at the very top. A small pipe is going down connecting it with a bigger size tank near the engine. I will try to upload more photos.

OK, uploaded! It is definitely an extra fuel tank.

Yes, the shape is similar to Pigeon! I believe it just does not look like just a mail carrier. I can see the round-shaped aim near the cockpit, which means the plane was likely designed for a battle.

What about the grid underneath the screw? Is this a sort of a radiator?

Beautiful find..., an original scale model from the 20s.
At first glance, the rudder is a typical "Sturtevant" design..., but the rest of the fuselage appears to be from another aircraft, or a type of Sturtevant that I am not familiar with. It could also be a free work, a personal design by the modeler... It looks like an old rudder concept for a "modern" fuselage design.

Sturtevant S-4
Sturtevant S-2

Some details suggest that the builder wasn't trying to faithfully model a particular airplane, working from detailed diagrams and photographs as one does nowadays a hundred years later, but was instead incorporating elements from various airplanes from personal memory.

peculiar arrangement of ribs in the fin/rudder

conical top and bottom panels of cowling, easily soldered from flat metal sheet but rare on manufactured airplanes

too-deep fuselage: the position of the exhaust stacks and prop shaft
makes one wonder what would have to be down in the belly.

A few more detailed photos (does the right side also have exhaust stacks?) would help to resolve this.


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How is d(Cl)/d(alpha)@stall and Cl increment to stall defined in Xrotor?
I hope there are some people around here with some experience in Xrotor than me. (Xrotor is:"XROTOR is an interactive program for the design and analysis of ducted and free-tip propellers and windmills." Found: here documentation is at the end of the site) For the Blade Section Properties, I need following:
========================================================================
1) Zero-lift alpha (deg):   0.00       7) Minimum Cd           : 0.0070
2) d(Cl)/d(alpha)       :  6.280       8) Cl at minimum Cd     : 0.150
3) d(Cl)/d(alpha)@stall :  0.100       9) d(Cd)/d(Cl**2)       : 0.0040
4) Maximum Cl           :  2.00       10) Reference Re number  : 2000000.
5) Minimum Cl           : -1.50       11) Re scaling exponent  : -0.2000
6) Cl increment to stall:  0.200      12) Cm                   : -0.100
                                      13) Mcrit                :  0.620
========================================================================

Now does anyone know how 3) d(Cl)/d(alpha)@stall and 6) Cl increment to stall is defined?
If I had to guess 3) is just the slop after Maximum Cl. But 6) which is also mentioned in the doc as "delta CL for the stall transition region" gives me some headache.
Any help is appreciated.
Cheers

I'm voting to close this question as off-topic because it is about using a software program, rather than about aviation, as defined in the Help Center.

@RalphJ This is indeed about software, but it's a software on the design of airplane rotor blades. Why are these types of questions not allowed? Where else would such questions be answered?

Indeed, and it's one of the basic, most popular software applications used in aerospace engineering. I would also consider a question about Xfoil acceptable.

@RalphJ I am new here and sorry if I posted this in the wrong forum but considering that this a software for propeller design, I thought I was in the right spot with aviation. As well as the tag propeller exists in this forum. On the other hand, if you are voting to close where should I post it in your opinion?

@LucEvertzen could you help me with that?

I interpreted the values as follows:

The angle of attack at which the specified propeller airfoil generates zero lift
The change in lift coefficient per change in $\alpha$ in radians
The same as 2, except that it is this value around the stall angle (it is mostly a safety factor for numerical calculations I think, small but non-zero)
Maximum $C_l$, which is achieved at the theoretical stall angle
Minimum $C_l$, which occurs on the backwards sweep of the propeller
The $C_l$ increment from the last linear part of the $C_l$ polar to the stall point
The $C_d$ at $\alpha=0$
The $C_l$ at $\alpha=0$
Quadratic drag dependence on $C_l$, slope of the $C_d,C_l^2$ curve
The Reynolds number at which these values are obtained
Re scaling exponent as explained in the XRotor documentation
Moment coefficient $C_m$
The critical Mach number.

Thank you very much for your reply! Now I have to get all this out of the poplars. One thing to 3) how did you define around the stall angle? I mean if you have maximum C_l then the gradient of the polar is zero. How far before or after maximum C_l do you get 3?

I simply used the stall angle or just a super small increment beforehand. I can't tell you with a 100 percent certainty but as far as I can remember the results weren't that sensitive to values such as 3).
If you want to be sure just play with the values a bit. Just increment these values by a small amount and see by how much your answer changes. If it changes a lot than we might need to take another look at it. Otherwise it doesn't matter as much.

Ok that sounds good for me. Thank you again!


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US ultralight licence for PPL pilot
I hold a European PPL(A) licence, and would like to fly an ultralight in the US.
In my home country, I'm not allowed to pilot an ultralight. To do that, I need a range of tests and a new licence.
What about the US?
Edit: I'm not going to a sparsely populated place like Alaska. What isn't clear to me is what's needed to fly ultralights in Controlled Airspaces or urban areas, as per §103.15 and §103.17
Edit2: I didn't realize there's a distinction between ultralight and light sports aircraft. What I want to fly is a LSA.

First, a caution about definitions. Some European countries define the word "ultralight" a little bit differently than the US. In those countries, "ultralight" means something more along the lines of what would be called a "light sport aircraft" in the US. Those countries typically use the term "microlight" for an aircraft that would be called an "ultralight" in the US. The CFRs you cited in your edit are for actual ultralights, but just to be safe, I'm going to answer for both LSAs and ultralights.
Light Sport Aircraft
In the US, an LSA is a one- or two-person aircraft that meets certain requirements. The only difference between an LSA and a normal GA aircraft is that someone who holds a Light Sport Aircraft License can only fly Light Sport Aircraft, while someone with a full PPL can fly both LSAs and not-LSAs.
Since an LSA is a regular aircraft, there are no special restrictions on where or when* it can fly. LSAs have the same radio and transponder requirements as regular aircraft for flying in controlled airspace.
* A Light Sport License prohibits you from flying at night, but that's a limitation of the license, not the aircraft. If an LSA meets the minimum requirements for night flight (lighting, electrical, etc.), a regular pilot could fly it at night.
Ultralights
In the US, an ultralight is a single-person aircraft that meets certain (far more restrictive) requirements. Ultralights have no licensing requirement, so you can fly one regardless of what kind of license you have, or even if you don't have any kind of license at all.
However, you're not allowed to fly an ultralight within class D or higher airspace, class E surface airspace, or over a built-up area**. You can get airspace authorization from the facility controlling the airspace, and a waiver for the built-up area restriction from your local FSDO, but you'd need to ask every single time you wanted to go flying, and it's not guaranteed. Much easier, IMHO, to cart your ultralight out to an empty field or untowered airport and launch from there.
** Read: Cities, towns, etc.

Thanks, LSA and UL are synonymous where I'm from. So to fly a LSA like the Tecnam, I can just use my non-US PPL licence?

@MrMartin Based solely on a glance through their Wikipedia page, yes, you could fly a Tecnam plane with a regular PPL.

You need to convert your PPL(A) to an FAA PPL in order to do so. Your PPL(A) allows you to fly aircraft registered in your own country in the United States, but does not allow you to fly N-registered aircraft in the United States. And if you're flying foreign aircraft using your foreign certificate, you need to follow its limitations, and that includes which aircraft you are allowed to fly.
Note that while it is called a "conversion," the FAA doesn't take your PPL(A) away, and you will have both licenses at the end. The process may take months though, and you will need an FAA medical as well.
From a proficiency standpoint, it would be wise to get some training anyway. Many people transitioning to LSAs find them difficult to handle, and especially to land. If you are just visiting the US and not moving there long-term, it would probably be easiest and safest to just pay a CFI to go up with you.


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Aerodynamics of Flight Control Surfaces
From what it appears to me, flight control surfaces seem to be taken for granted. In terms of how they actually work and what kind of physical outputs they give in terms of forces and such.
Take for example the elevators of a Boeing 777, if the pilot wanted to pitch up, the elevators would move up. What are the physics behind this? Is it air being deflected thus a change in momentum occurring to pivot the aircraft's tail down or is the angle of attack changing due to the moving nature of the trailing edge? or both?
I am quite interested to know because I want to simulate Aerodynamical forces in a game I plan to make soon. Diagrams would also be useful but not required. Thanks in advance.

Welcome to aviation.stackexchange! These are all great questions and very much on topic here. Generally there is a "one question per post" policy here so you may want to split these apart into 3 posts to get better answers.

Alright, I understand. Thanks.

I hope you learn quickly because you are going to have to move pretty fast from grasping the basic physics of how a control surface changes pitch, to understanding the mathematical equations well enough to write accurate simulation code.

You may transform the title in a question (this is a Q&A website) as specific as possible to help navigate through questions about aerodynamic or surface control without opening each question.

Thankfully, aerodynamics in the usual flight range is linear.
Therefore, there is a gradient of lift over angle of attack and another one over the flap deflection angle. Both are constant over a range of maybe ±15° and can be combined. The angle of attack is referenced to the fixed part of the flight surface and the deflection angle to the moving part relative to the fixed part.
Another parameter which influences lift is the camber of the flight surface. Positive camber produces more lift at the same angle of attack. Deflecting a flap changes this camber, and its effect can be linearly added to that of the angle of attack.

Is it air being deflected thus a change in momentum occurring to pivot the aircraft's tail down?

Yes, when a flap moves, the angle of attack stays constant but the camber of the flight surface changes, thus producing a change in lift. In your example, a trailing-edge up deflection decreases camber, the surface produces less or negative lift which produces a moment around the center of gravity.
And yes, lift is produced by deflecting a stream of air.
Things get more complicated and interesting when the aircraft leaves the angle of attack range in which aerodynamics is linear.

Ah thanks, if it were flaps on the main wing for landing lets say, would the direction lift produced by the main win change to be more opposing to forward movement but still enough to maintain stable flight? And by deflecting you do mean that air is deflected over and under the wings right?

Lift is not linear. Especially once it reaches the stall angle of attack.

@MarkJonesJr. Yes, as this answer clearly says. What is wrong with that?

@PeterKämpf the first sentence in your answer says "Aerodynamics is linear." Clearly.

@MarkJonesJr. No, it doesn't. Please read the full sentence.

A trailing-edge control surface, when it deflects, changes the camber of the overall airfoil.  More camber means more lift, in whatever direction that airfoil is mounted.
In your example, adding up elevator increases the horizontal stabilizer's camber, which increases the downward force it applies.
Philosophically, "why" it does this is just, well, that's how air behaves when you push it past something with that shape.

From an old uni book on stability & control of aeroplanes, showing the pressure distribution over the horizontal tail:

First picture: no elevator or trim tab deflection, pressure peaks at the stabiliser nose at two different Angles of Attack $\alpha_h$.
Two different elevator deflections $\delta_e$: pressure peaks at the elevator profile nose.
Two different trim tab deflections $\delta_{te}$: pressure peaks at the trim tab nose.

The magnitude of the pressure peaks per degree deflection is a function of the surface area. Location of the peaks is a function of which surface has AoA ≄ 0.

What is this graph against?

@Kiyo The length of the horizontal tail chord.

Well, both.
Lift can be described as a moving wing colliding with air molecules at an angle, the result of the collision is the wing moves one way and the air mass the other, as per momentum physics.
Moving the trailing edge, or the entire surface, increases the angle of attack, resulting in more lift at a given speed $V$:

$Lift$ = 1/2 × Lift Coefficient x Air Density x $V^2$ x Wing Area
Deflection of the control surface produces a linear response to lift, as Peter Kampf says.
A graph of angle of attack vs lift coefficient generally shows a linear  relationship through most of the unstalled AOA range.
Further inspection of the effect on AOA, of deflecting a trailing edge flap down, as compared with "drooping" a leading edge, does indeed show doing this changes the angle of attack of the lifting surface!
Flaps are usually near the wing roots, and leading edge slats near the wing tips, for this reason.  We want the root to stall first.
Yes, deflecting a control surface changes the camber of the wing, which also factors into the lift coefficient, but,  relative to the original AOA of the wing/fuselage, the flapped portion of the wing will stall at a lower AOA than the slatted portion.  Therefor, deflecting the control surface also changes AOA.
The importance of this concept is highlighted in a slow flight coordinated turn.  Use of ailerons without coordinating rudder may result in a sharp roll in the opposite direction of the intended roll, because the AOA of the "down" aileron wing now exceeds its stall AOA.

A wing colliding with molecules is a (useful) simplification, like an angled steel plate encountering a fusillade of bullets.  But those molecules colliding with all the other molecules is what causes lift.  Avoid the bullet imagery.  Safer to not mention molecules at all, and instead stick to pressure differences.

Momentum theory describes "surfing" the air (bottom lift), but not "Bernoulli" pressure differences (top lift).  Bernoulli is limited at both very high and very low Reynolds numbers, but would be applicable for most aircraft control surfaces such as horizontal stabilizer/elevators.  Momentum theory need not involve bullets, it is useful to describe the properties of collision, and is part of "lift". But what works for the bird does not work for the bee, or the blue whale.  There is no ubiquitous definition for lift.

Momentum theory only gives useful results at hypersonic speed when inertial energy dominates the flow. As Camille says, better avoid it for describing subsonic flow.

@Peter Kampf your "constant over a range" link clearly shows the "Bernoulli lift spike" (with much lower drag) in the Coefficient of Lift vs AOA graph until stall.  Post stall, lift drops with higher AOA, but then rises again to a second maximum at 45 degrees (with hugely more drag).  Can not this second maximum, like a water ski, be entirely explained by momentum theory?

But, most importantly, we all should be aware that a control surface, flap, or slat  deflection can locally change AOA.

Yes, the induced AoA. Per definition, the "real" AoA s between the wing chord and the flow direction at infinity. Now to momentum theory: At a very macroscopic scale the answer is a qualified yes, but if you look closer, all vorticity is left unexplained by momentum theory. Since there are better theories, I prefer to leave momentum theory to hypersonic speed and interactions between satellites and very rarefied air.

Agreed there, and as Camille says, interaction of air molecules with other air molecules (viscosity) will play a role too. (Perhaps more so with water?).  Maybe the "Lift coefficient" can be refined a bit to AOA × airfoil type (both linear). Thanks.


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Why do helicopters crash in low visibility (IMC)?
There are no known direct eyewitnesses of the crash of helicopter flight Sikorsky S-76B N72EX with 9 people on board, including Kobe Bryant. However, there are many earwitnesses who heard the helicopter flying over their heads, but weren't able to see it due to extreme fog at the site, shortly before it crashed into a hill. Therefore, extremely low visibility was the probable cause of this helicopter crash. By assuming that, here's the question:
Why do helicopters crash in low visibility (instrument meteorological conditions IMC)?
For instance, what are current theories about the sequence of events of the helicopter crash of flight N72EX, assumed the crash happened due to the low visibility on site? Why did the pilot suddenly increase altitude above Ventura Freeway 101? And why didn't he follow freeway 101 by GPS (on this high altitude), although he was a certified IFR pilot flying under special VFR? And after his ascent, why the slow turn to the left followed by a sudden increase in speed while diving into the hill at about 4000 feet per minute? (Primary source: Flightradar24) (Note: ATC recording doesn't provide much useful information...)
[Note: I'm not interested in the many different theories or speculations about the crash of flight N72EX, just what are the most probable events that lead to the crash, if it was indeed caused by low visibility? Btw the crash flight N72EX was just used as an example. Actually, any other helicopter crashes caused by low visibility could be described in details as well, if they contribute to answering the main question. Please answer by using primary and secondary sources as well as some logic, physics/aerodynamics and psychology, thanks.]

That's quite a bit of assuming, generalizing and speculating...

This could be improved by focusing on the actual question in the second paragraph. Though you say you are not interested in speculations, the rest of the post seems to be mostly focused on it.

Please consider FAA and aeromedical sources on physiology, aerodynamics, disorientation, etc before asking us to reproduce years of exhaustive studies on the subjects. There is a lot of information out there on what causes disorientation (physically, mentally, medically, etc) that we don't need to reproduce here, and wouldn't fit the format of this site anyway.

Why do any aircraft crash in low-visibility IMC?

@fooot: I am considering to change the question or maybe the title. But what I actually wanted to ask, why is a fully certified IFR pilot flying under special VFR not capable to fly in a fog or cloud? I just can't understand it. The media is always talking that the helicopter flew under VFR, but that was not the case at all. All the assumptions and speculations are about a VFR helicopter flying in IMC conditions, but this is simply plain wrong...

@Sean: Yeah, why do IFR planes crash in IMC? That's exactly my question...I am considering to change it.

@FlyHighJess - Primarily this one:
https://aviation.stackexchange.com/questions/73877/are-helicopters-impacted-by-fog-more-than-airplanes  But also this to address why he couldn't just stop:  https://aviation.stackexchange.com/questions/73857/can-all-helicopters-hover-and-how-long-does-it-take-to-stop  There's more on the challenges of IMC flight in general:  https://aviation.stackexchange.com/questions/31198/why-does-flying-ifr-require-a-rating

Hello everyone, I completely changed question and text accordingly, was a lot of work. Thanks especially to @Sean and "fooot" to get my thoughts straight...hope that the question will be unlocked :-)

Much better, but I still think that waiting on the NTSB report is a better bet than asking random internet people to speculate.  Good enough to vote for unlocking though.  And i deleted my earlier comments, leaving up the links.

Since this was closed, i'll answer here. Flying is not natural for humans. Flight operations can induce you to feel you are going straight when you are turning. when you are flying and don't have a ground reference it is easy for you to think the instruments are wrong because your inner ear is telling you the world is fine. The instruments are correct and your ear is wrong. Most helicopters are not IFR rated even if the pilot is IFR trained. IFR in IMC requires regular training to maintain proficiency over the minimums to maintain IFR status. this comes up in every IMC crash.

Spacial disorientation doesn't just happen in IMC. On a dark night(no moon) This pilot lost spacial awareness when going from lit gauges inside the to poor lighting outside. Night vision would have been destroyed. and with no lights outside to allow visual spacial orientation it was completely lost causing a multiple fatal crash. https://www.atsb.gov.au/publications/investigation_reports/2011/aair/ao-2011-102.aspx

The only flight rule that will completely eliminate human error is to ban people from flying planes or programming UAVs.
More regulations won't solve this.  Aviation is already strictly regulated.
As for why the pilot did not climb, well, you will have to ask him.
Keep in mind that a mechanical failure could be to blame.
We just don't know.
Answer to the previous question:
The risks of continued visual flight into poor conditions are already pretty well documented.  It is a serious hazard for all flying machines, not just helicopters.  We don't know if that is what caused the crash or is even a contributing factor, though some of the makings of a CFIT accident are present (bad weather, possible perceived pressure to go, and possible complacency due to familiarity with the area).  Anything we say here, including what I just typed, is pure speculation.
We don't know why the helicopter crashed and we will likely never know for sure.  It was not equipped wIth a CVR or FDR nor was it required to be so equipped.
This is a tragic accident and that's all we know.

Yes, we don't know what caused the crash, but as I wrote I assumed it's due to poor visibility. Now, why is it dangerous to fly IFR without any visibility? Because the pilot was IFR certified, and flying under special VFR. He should have been perfectly capable to fly through the fog by ascending and then just navigate by GPS.

You are correct in a sense. If you are IFR rated, have a clearance, and commit yourself fully to flying and navigating by instruments only it is fairly routine.  The trouble is when you aren't legally on an IFR clearance.  I am not a helicopter pilot, and have never technically flown special VFR, but I know how challenging it can be to be navigating VFR in poor conditions and trying swap your attention between terrain and instruments.  The inside/outside scan transition chews up time and degrades rather than enhances situational awareness.  You have to trust that the pilot was doing his best.

@FlyHighJess Nobody disputes that a professional pilot should be able to handle an SVFR clearance and flight.

@acpilot: I almost completey changed the question. Would be great if you'd like to update your answer accordingly, thanks!

Your above response sounds like you are assuming that IFR skills and GPS alone are enough to keep you safe in IMC. The safety does not start there. It starts with the IFR flight planning. Proper flight planning would NOT have included flight that close to terrain. All IFR flight planning includes terrain clearance in excess of 2000 feet unless specifically on a published and defined flight path on a published Instrument Procedure. This IFR capable pilot was not flying IFR. He could not fly IFR because he was not flying on an IFR flight plan.

If your questions on this site are meant to improve the safety of the flying community, I applaud you. We should look at this incident as a learning opportunity. The lesson learned is one of personal responsibility as pilots. In IMC, the IFR pilot had the legal  responsibility of flying above the published IFR Off Route Obstacle Clearance Altitude of 8900 MSL except for immediate takeoff and landing under visual conditions. He had a personal responsibility of staying above the Minimum Elevation Figure of 3200. Lack of that altitude caused the crash.


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Why are there no emergency flight rules in place to prevent IFR-certified helicopters, operating under special VFR, crashing in low visibility (IMC)?
In modern times, engineering allows humans to do everything: Drive in fast cars or trains, diving into the deep sea, and even going to space. Which is a great thing indeed.
But when it comes to technologies or rules on which life really depends on, it fails too much. I refer to the crash of helicopter flight Sikorsky S-76B N72EX with 9 people on board, including Kobe Bryant:
The helicopter was flying under special VFR and the pilot and aircraft were fully capable of IFR flight, according to this source: "Special visual flight rules blend the two: If you’re passing through an area enforcing instrument rules, but are heading for an area under visual rules, you can ask to continue flying under visual rules, so long as you stay clear of clouds. It’s fairly common but is allowed only for pilots who are certified to fly under instrument rules, and only in helicopters with the needed instruments. Pilots sometimes use the special visual flight rules to avoid the hassle of filing detailed plans before takeoff, required on flights under instrument control."
That being said, upon reaching the heavy fog of the Santa Monica mountains, the pilot could have easily switched to emergency IFR. Because special VFR aircraft need to be fully capable for IFR, just in case "something went wrong".
As a matter of fact, upon reaching the Santa Monica mountains the visibility was so bad, that the pilot of flight N72EX did indeed start a steep climb and flew through the clouds, therefore at instrumental meteorological conditions (IMC) which forced him to go into de facto IFR mode, obviously, which would not have been a problem at all.
Therefore, the extremely low visibility was not the (direct) cause of this helicopter crash, but rather a too low altitude, which has been recognized by the responsible ATC (have a look into the ATC recording starting at about 4:00 until the end). There are no known direct eyewitnesses of the crash of helicopter flight, but there are many earwitnesses who heard the helicopter flying just over their heads, but weren't able to see it due to extreme fog at the site, shortly before it crashed into a hill, therefore the altitude was too low indeed.
So how is it possible that in the 21th century a fully capable IFR helicopter and a IFR-certified pilot are making such an error, trying to navigate between hills by GPS while maintaining an altitude much too low for IFR or flight following?
Why isn't there an emergency flight rule in place that would have allowed the pilot with 20 years of flight experience to climb right before the Santa Monica mountains until the IFR minimum ground altitude was reached (maybe around 2400 feet), continuing the flight under emergency IFR and by flight following?

If a downvote, please comment why, thanks!

"Why are there no rules to prevent crashing..." - really? Think about the question you're asking here. Of course there are rules to prevent crashes. (Typically they do that really, really well.) Or is the question why don't those rules work perfectly, preventing every crash... because perfection, when dealing with people, is unattainable.  This is essentially about speculation on a recent accident, which is explicitly off-topic here. Voting to close as such.

I'm voting to close this question as off-topic because it is essentially asking for speculation about one particular aircraft accident, which is off-topic.

No, the rules aren't perfect at all, otherwise they would have prevented a certified IFR-pilot from hovering just above the ground through mountains in heavy fog, although he was IFR-certified., Upon realizing that the fog doesn't clear but became thicker and thicker, he should have been able to switch into emergency IFR mode and then ascending over 2300 feet which would him have cleared from all the mountains. Then flight following to get him through the clouds...he might have survived

@Ralph: I changed a question a bit. But now I think this was enough for today...

I feel like this question is related: Can legislation help prevent CFIT or will it restrict freedoms too much? (closed)

“As a matter of fact, upon reaching the Santa Monica mountains the visibility was so bad, that the pilot of flight N72EX did indeed start a steep climb and flew through the clouds, therefore at instrumental meteorological conditions (IMC) which forced him to go into de facto IFR mode, obviously, which would not have been a problem at all” - we don’t know for sure at this point that that was the cause of the final abrupt maneuvering or the crash.

@FlyHighJess - your statement about “the rules aren’t perfect at all” is correct. They can not be perfect. The people writing them aren’t perfect. Neither are the people following or not following them. This pilot did not follow the rules. If this were a true emergency as opposed to poor ADM, the regulations actually allow for the PIC to deviate from the regulations for the sake of the preservation of life. Any way that you change the regulations to avoid this type of accident, you will have pilots that try to push that envelope to the point of catastrophe.

Bottom line is that this pilot did not follow already established regulations. He attempted to skirt the regulations just like he was skirting the cloud layer and fog. Just because the regulations are there, it does not mean that people will always obey them. You can change the regulations all you like. You will not change the people. And, this is even considering that pilots are some of the most conservative, safety conscious, calculated, pro-risk-management people in society.

The aircraft had passed into Class G airspace well before crashing and thus could not have still been flying under a Special VFR Clearance.

@FlyHighJess - The published mandatory IFR Minimum Off Route Obstacle Clearance Altitude in that area is 8900 feet MSL. The published recommended Minimum Elevation for safe flight in the best of conditions is 3200 feet MSL. Except for immediate takeoff and landing, the pilot should have been above one of these two altitudes (depending on flying in IMC or VMC). Regardless if the pilot had a filed plan or made an emergency decision, he was not flying at the sufficient legal nor the sufficient suggested altitude for the conditions. He had both a legal and a personal responsibility to do so.

There are rules. Without going into too much detail...
As you probably know, IFR flight requires the pilot to file and receive clearance for their flight plan. If you are already airborne, you are required to stay clear of clouds and have 3 mile visibility (1 mile for SVFR) until you receive clearance for an IFR flight plan. That flight plan is going to require you to have at least 1000 feet to 2000 feet of terrain and obstacle clearance depending on if it is in a mountainous area. Filing VFR does not have those restrictions. It also allows helicopters to get closer to terrain and obstacles like buildings so that they can prepare for landing in those areas. The pilot is required to do so in a manner that does not endanger persons or property.
In other words, a helicopter can fly at extremely low altitudes while under VFR. It can fly close to terrain under VFR. That is the special purpose of a helicopter. It is the pilot’s responsibility to discontinue the flight when the conditions are no longer VFR. The alternative is for the pilot to file a pop up IFR plan, and remain VFR in VMC until receiving clearance. Otherwise, he is breaking already established rules and best practices.
Problems arise when pilots fly at the edge of VFR when the conditions worsen or the terrain changes. A similar analogy is that you can drive your car at exactly the posted speed limit and be both safe and legal. If the roads become slippery due to rain, snow, or ice, you will be in violation of “careless or reckless” regulations.

So in other words, the rule are set up in a way that actually promote crashes rather than preventing them? Because if the pilot would have been gone into full IFR, rising high about the mountains while requesting flight following, he would have broken established rules and best practices, but he would have survived nevertheless. Instead he tried to hover right above the ground and got himself and his 8 passengers killed. What kind of "best practices" are these, if the don't consider a violation in order to survive in an emergency situation?

Maybe the rules need to set up differently between clouds and fogs, because clouds are easily visible an can be avoided by just not flying into clouds. But when you actually fly into fog, it's possible that the fog suddenly vanishes and you're cleared, but it's also possible that the fog gets worse and worse leaving you with zero visibility. That's what happend in L.A. last weekend, and I strongly think that the rules concerning fog needs to be changed for the saftety of air travellers, somehow.

To my understanding, the pilot did not hover in VMC. Hovering in VMC is difficult for a newbie. It is old hat for a veteran pilot. Hovering in IMC is way more difficult.

The FAA regulations are set in place to force pilots into making the safe, right decisions. Pilots are required to know the weather at their point of departure, destination(s), alternate landing zones, and enroute. The FAA regulations state that you have to have alternates to your plan, whether VFR or IFR. The regulations set in writing what you have to do and how you have to do it to fly in IMC. The all of the regulations culminate in making the right decisions BEFORE you even start the engine.

Again, just like my analogy about driving, the regulations promote safety. They do not guarantee it. There is no way to do that. Even if they did guarantee safety, they can not guarantee that the pilot/driver would follow the rules. Have you ever driven 1 mph over the posted speed limit? If you have, you have broken a regulation. Have you ever driven in bad weather. If so, common sense as well as the regulations state that you must drive below the posted speed limit.

@FlyHighJess Fog is a cloud, and pilots already can't legally fly into either, except under IFR, for exactly that reason. Being illegal doesn't stop people from occasionally doing it, though.

The aircraft had passed into Class G airspace well before crashing and thus could not have still been flying under a Special VFR Clearance.  Also, 3 miles visibility is not required in Class G airspace.

@FlyHighJess - if your questions on this site are meant to improve the safety of aviation, I applaud you. The lesson learned in this crash is the personal responsibility too follow the existing regulations. Some of the current regulations for that area are: The minimum IFR Off Route Obstacle Clearance Altitude is 8900 feet MSL; The recommended Minimum Elevation  for safe flight is 3200 feet MSL; There is a Temporary Flight Restriction around the crash site. If I decide to fly through the area right now, I would be violating current rules, regulations and safety procedures.

@ Dean F.: You said IFR requires 1000 to 2000 feet terrain clearance. But in the comment of the question itself you wrote: "The published mandatory IFR Minimum Off Route Obstacle Clearance Altitude in that area is 8900 feet MSL. The published recommended Minimum Elevation for safe flight in the best of conditions is 3200 feet MSL. Except for immediate takeoff and landing, the pilot should have been above one of these two altitudes (depending on flying in IMC or VMC)." - How much is it in case of IFR for that particular area: Around 2500 + 2000 = 4500 feet, or really 8900 feet for helicopters?

@FlyHighJess - 91.119 states that an aircraft can fly 1000 feet above a congested area, or gathering of people, or 500 feet above water or sparsely inhabited areas except... “(d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—
(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA;”

It also states that you have to be at “ An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.”

The published Minimum Elevation Figure (MEF) gives you the MSL altitude of the top of any point, natural or man-made in that VFR Sectional quadrant plus an error factor. If you are flying through a quadrant at high speeds or marginal visibility, or you are unsure of your exact location, the prudent and recommended course of action is to be above the MEF. The IFR OROCA takes into consideration the MEF and IFR airways and flight paths for traffic clearance/separation in that quadrant. If you are totally IMC, in an emergency, and off Route. the OROCA gives you a kind of safety net, so to speak.

Well the flight in question was operating under VFR nor had the pilot filed an IFR flight plan.  The flight in question had an off airport destination which would have only allowed for a descent and landing under VFR operations.  Could the pilot have filed an IFR or composite IFR/VFR flight plan?  It’s possible.  But the descent and landing would have had to be done in VMC as there are no available helicopter instrument approaches available for an off airport site like that.
VFR helicopter flights in marginal weather are not that unusual.  And, given the pilot did these kinds of flights on a regular basis, there was no reason to think the day in question would be any different.
Third, N72EX WAS flying on an SVFR clearance and did have flight following at the time of the accident, so they were in contact with ATC at the time of the accident.
Again as to why the pilot did not file IFR for a route out to a Thousand Oaks is unknown.  As stated above, it was an off field landing so an instrument approach was not available.  The WX allowed for VFR for a helo that day.  The pilot had flow those kinds of routes before and may have felt confident that he could do it again.  It may well come down to mitigating factors such as poorer weather than anticipated, a flight crew and charter firm fighting schedule pressures to keep a client happy (Bryant was flying up to a sports park he ran in TO to guest coach a basketball game his daughter was participating in that day).  And, finally, it might also be a case of otherwise smart people doing foolish things in aircraft and paying the price for it.  This FAA film with real life helicopter pilot Clint Eastwood talks a little about contributing factors and how they can lead to dangerous situations.

Until we get more details, this kind of talk is just Monday Morning Quarterbacking; “If I were up there, I would have done so-and-so and would have never crashed”, etc.  That’s easy to say on the ground at 1G; many benign situations can become death traps and its insidious just how fast that can happen.
As to intentionally entering IMC while flying through a canyon tracking a freeway visually at 130+ KIAS would have been extremely dangerous.  While you might be able to justify that under the authority of PIC under 91.103, a CFIT would be highly likely.  Even if you could climb out of there, entering controlled airspace in IMC without a filed IFR flight plan and without a clearance in congested SoCal airspace would also be a very risky gamble.

The aircraft had passed into Class G airspace well before crashing and thus could not have still been flying under a Special VFR Clearance.

At the end of the flight, yes, though the controllers did not terminate SVFR nor did the pilot request to.

Good argument @CarloFelicione: "The pilot had flown those kinds of routes before and may have felt confident that he could do it again." - I also think that the pilot's experience has worked rather against him than benefitted him. As in "I've flown this route already a hundred times, so I know exactly what to expect." So even if he had a rotor failure, by flying only 2000 feet with instruments only he was taking a certain risk into account despite his IFR-capability.


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How different is ADFX/ARINC 664 router from normal Router?
ADFX or ARINC 664 is a protocol that is derived from the Ethernet protocol. What I understand it is

Point to Point (no routing of data)
Deterministic
used VLAN for addressing

I just wonder do we have to get specialised router that cater to ADFX? Can the conventional ethernet router/switch(for eg used in home, office) be used instead?
Regards
EDIT: Sorry based on answer below given, I would like to edit my question to being instead of routers to switch

This question would be better suited on serverfault

@ManuH I'm not sure that it would, as ADFX or ARINC 664 is probably off-topic for serverfault.

@selectstriker2 serverfault is better suited for network protocols (this seems to be a question about physical layer (layer2) and network layer (layer3) of a specific network protocol)

But it is specific (exclusive?) to aviation, not something typical in the commercial world.

@ManuH, ARINC 664 it is specific to aviation. There is a similar standard for automotive and railroad and industrial probably have their own variants by now, but it is never ever used in general computing.

@JanHudec OK, good overlap between both site. Let's be proud of our community and keep the question here.

The point of ADFX/ARINC 664 is that it is real time. That means the switch guarantees each type of message will be delivered in specified time according to its priority and allocated bandwidth.
For example the control computers probably need to know the inertial reference (attitude and speed) readings every 10 ms, so the switch can be configured to deliver one message from each inertial reference unit every 10 ms with at most, say, 2 ms delay (based on number of packet types with higher priority). And the switch guarantees that barring hardware failure it will deliver the messages within that delay always. That is a lot of logic on top of normal ethernet that just delivers packets on a best effort basis.
Additionally the logic also has to guarantee that if one of the units goes haywire and starts sending the packets faster than the allocated bandwidth, or starts sending jumbled packets, these errors won't interrupt any other communication going through the switch. If one inertial reference packet should be delivered every 10 ms, only one will be delivered even if the unit sends hundred of them.
And last there has to be a lot of integrity checking of the switch itself so if hardware does fail, the communication fails over to the backup network.
Since it is ethernet on the physical and data link layers, you can use plain ethernet switch for testing a component. But not for production and not for system test where you have to verify the traffic policing is set up correctly.

I haven't worked with ARINC 664/ADFX specifically, but my understanding is that aircraft use specific ADFX switches.
From Wikipedia:

Since AFDX utilizes the Ethernet protocol at the MAC layer, it is possible to use high performance COTS switches with Layer 2 routing as AFDX switches for testing purposes as a cost-cutting measure. However, some features of a real AFDX switch may be missing, such as traffic policing and redundancy functions.

Since ADFX and ARINC 664 are Layer 2 protocols, and routers operate at Layer 3, the answer to the question

How different is ADFX/ARINC 664 router from normal Router?

is easy. There is no such thing as an ADFX/ARINC 664 router, just like there is no such thing as an Ethernet router, since routers operate at Layer 3, not Layer 2. The question how something that doesn't exist differs from something else that also doesn't exist is meaningless.

W Mittag. Sorry I would mean switch


text:
I want to be a French fighter pilot. Any advice?
Hello everyone!
I want to be a French fighter pilot and I would like some advice from retired fighter pilots about how you should do things if you were at the same age as you were when you entered the army but having all the knowledge you have now.
Thanks for reading. If you see anything that can be improve (even the question), let me know! It's the same for questions or/and remarks, let me know!

Contact the french air force They have several recruitment path and have a dedicated service to answer those kind of question.

Dear @O'Schell, your question seems to be odd for Stack Aviation. I suggest that you consider querying about objective and not opinion-based subjects. In your case, you may approach the question by asking what are the official French fighter pilot requirements. I am sure that plenty of people in this community will help you out.

Unfortunately we don't have as many current or retired fighter pilots as we used to, I think this is opinion based and not a good fit for StackExchange. If you can narrow this down to a fact-based question it would fit better here.

@O’Schell - some suggestions to narrow the focus of your question might be - “What can I do now at my age/education level to prepare...?”  Or - “What is my next step to...?”  Or - “How can I better my chances to be selected to...?”  Or - “How beneficial would it be for me to...?” Who, What, When, Where, Why and How questions may get you a better response.

When you write "learning by hearth" I assume you mean "learning by heart", but that can mean a number of things. Do you mean "being passionate about learning" or do you mean "rote memorization"?

@ DJClayworth More like "being passionate about learning", if I understood right. I don't like loving to learning things(like long list of word that makes non-sens) but I like learning what I like (like math formulas, aviation things, programmation things), even if they are longer.

Your question was closed because it is considered as too subjective by the community. I strongly encourage you to modify it as Dean F. suggested so that it is obvious that the answers apply to everyone and not just you.

@O'Schell - Just FYI, in English, “Learning by heart” for the majority of readers will not be interpreted as passion. You will need a lot of passion while learning to fly. Because, you will have to learn a lot of things that you will not be passionate about. Here is a list of items to go study. If you can find within yourself a passion for learning these items to the point of near memorization, you may make it as a pilot.

The codified list of aviation regulations for your country (the FAR/AIM or Title 14 CFR in the U.S.). 2. The POH/AFM for the most popular training airplane in your area. 3. An actual checklist for inflight use for said training aircraft. 4. A life-sized, detailed poster of the cockpit (specifically the panel, pedestal and overhead console) of said aircraft. 5. Stick and Rudder by Wolfgang Langewiesche. The first 3 of these can be found for free online in the U.S.

Sorry, but this website is not well suited to provide advice or suggestions, that's why this question was put on hold. If you can rephrase it to ask for specific factual information, please do. Otherwise we won't be able to help you.

Be sure to start by learning to speak French.

@quietflyer I'm French, actually...

The third point in your original list is a clear red flag:

I'm a huge fan of airplane (but I'm not the type of guy loving learning every thing by heart)

I don't mean to put you off with what follows, but honestly, you've got to love learning everything by heart in the military in general, let alone the airforce.
Becoming a professional pilot is demanding, becoming a military pilot even more so. Both require high intellect, the ability to learn and memorize things easily and preferrably, especially in military aviation, a burning desire to do so.
On top of that, to make it as a military aviator, not only do you have to be enthusiastic about aviation in general, but you have to have just the right mindset: utmost dedication for the military way of life, deep patriotism, and absolute altruism. Any succesful military person is first and foremost a soldier, only in addition to that can one be something more.
The guys and gals at military aviation operate on a whole different level from an average pilot. You simply are not going to make it if you are not all in. About aviation and more importantly about military.

Hooowaaah, Airborne!! Well said. Now, go do push-ups till I get tired.

Yes, being 100% dedicated sucks. Studying sucks. Learning things by heart sucks. Things not coming to you easy sucks. Embrace the suck!!

Oops, just realized you were picking on me for referencing the wrong point @DeanF.  Fixed now...

Yes, I am shameless.

No, wait, the Q was edited 'bout the same time I answered... well, anyway, matches now.

"Learning by heart" does not usually mean "learning with passion". It can also mean "learning by rote".

Either or, you absolutely need both where the asker is aiming.

Study. Study. Study
I am not sure if the phrase “learning every thing by hearth” in French is the same as the English phrase “know by heart”. I figure that your question just has a typo. You will need to learn to love learning.
Admittedly, I only know American military pilot experiences second-hand. But, I have flown with military pilots on the civilian side. What will be common in aviation in general is that you need to love to learn. And some of that knowledge, you need to learn to know by heart. Just remember, it’s not impossible. Millions of people have done it before you. You, also can do it.
Almost everything you do in an aircraft has a checklist. That checklist combined with the rest of the AFM and supplements is your airplane bible for the moment. There is very little you should be doing that is not found in a checklist. I often tell new pilots to go through the full checklist even when they fly the simulator or chair fly.
Most of the checklist is a “Do then Verify” list. That means you will have to memorize a lot of procedures so that you can perform the memory items when speed is of the essence. You use the checklist to make sure you have done everything to standard and not missed anything.
This is just one example of knowing it by heart. Committing things to memory and muscle memory. Practicing and rehearsing and practicing and rehearsing. And when you are done practicing and rehearsing, practice and rehearse some more. A lot of civilian pilots have a background with that type of discipline. People who have excelled at a musical instrument comes to mind.
You can teach yourself that discipline by practicing it everyday in whatever you do. Build great study habits and stick to them. Practice deliberate and precise driving. Refine your fundamental skills and techniques in whatever sport you enjoy. Or, figure out a way to start flying now. Be purposeful and diligent in even the little things in life. No, especially the little things in life.
Hope this helps.

The thing about military way of practising and rehearsing is, that it more often than not includes some sub-par elements deliberately induced into the process

It’s called training. And, training doesn’t start until someone is miserable.  BTDT.


text:
Visualising downwash over a finite wing
The downwash is caused due to higher pressure below the wing than above it (for a positive lift generating wing that is). What I am not able to understand is how would the flow be in a gross sense? Will the flow move bottom to up around the wing tips and move top to bottom along leading and trailing edge? I thought of this because it is the only way I am able to justify that downwash induces a negative velocity component which decreases the angle of attack that the wing sees at the leading edge.
But can’t the air also move bottom to up along leading or trailing edge? To me, at leading edge it seems unlikely based on intuition because of the air upstream constantly trying to direct the air ahead but at trailing edge it seems more likely.
So what is the correct visualisation of downwash over a finite wing?

You may find interesting information that helps you improve your understanding in the section airfoils and airflow of the site How it flies?

Your question is about circulation, not downwash. see https://aviation.stackexchange.com/questions/47251/what-is-the-physical-meaning-of-circulation-found-in-kutta-condition

see also how wings generate lift  https://aviation.stackexchange.com/questions/16193/how-do-wings-generate-lift

@Pilothead The direction of flow that OP has in his/her mind would be in the opposite direction of the circulation.

@jzyl Air does not flow in the opposite direction of circulation, regardless of the OP's question.

Downwash is a misleading concept when discussing airfoil lift. Instead, just focus on the pressure distribution around the airfoil.
If I understand your confusion correctly, you are picturing, correctly, that there is an overall low pressure on top and an overall high pressure on the bottom, then why isn't air flowing from high pressure to low pressure?
This will indeed be the case, if there is no airflow. What you are missing is that, the static pressure distribution is caused directly by the airflow around the airfoil. The distribution is steady-state, because the combination of dynamic pressure and static pressure resists further changes; it's at an equilibrium.

I am quite confused about the use of static pressure in this picture.  Can you kindly more on the questions below?
'The static pressure distribution is caused directly by the airflow around the airfoil.

@Niket I'm not sure I understand what you're confused about. The "pressure" from your original question is equivalent to static pressure. My answer just says that there's also dynamic pressure that resists changes to the flow at steady-state.

I am sorry but I am still not able to get it. Is the static pressure distribution something that is always there and then when the airplane is passing through that region , this also comes with it some dynamic pressure. Now if I understand you correctly, the initial static pressure distribution and the newly introduced dynamics pressure distribution get into an equilibrium. Since all this has happened and the air  is in equilibrium, we don't have much (if at all any) air that moves top to bottom  or vice versa on the wing. Is this true?

The downwash is caused due to higher pressure below the wing than above it

No, not really. Actually, the wing sucks and pushes air down so it leaves the trailing edge in a different direction compared to the flow direction ahead of the wing. Air that flows over and below the wing gets a downward impulse that leaves it moving downwards behind the wing. That movement is downwash.
The low pressure area over the forward upper wing does indeed suck air upwards into it. The flow ahead of the leading edge is sucked upwards and once the lower part of it has arrived on the bottom of the wing, it will not flow back – that would require a complete change in flow direction and costs too much energy!
High and low pressure alone will not cause a change in movement, only a pressure gradient will. The stronger that gradient, the more sudden the change in movement. A complete reversal of the direction of flow would require a very high gradient which can only be found in places like the inside of engines but not around a wing.
Note that a wing has to move through air in order to create those pressure gradients, and their strength increases with the square of velocity. Therefore, as JZYL explains, you need to see the flow from high to low pressure in combination with the general movement of the wing.
For more details, please read this answer.

Could you please answer these follop-up questions?

So, is it right to say that  the phenomenon of air being pushed downward is dowmwash?
High and low pressure alone will not cause a change in movement. Why is it so?


text:
How to find the static stall angle of attack for a given airfoil at given Re?
For vertical axis wind turbines applications
I'm wondering how one would find the static stall angle of an airfoil at a given Reynold number?
Is it just the critical angle of attach (AoA) where highest lift is achieved or some value beyond that?
Consider the given graph for NACA0012 (for the use of vertical axis wind turbines) at Re of $3\times 10^5$, what will be the static stall angle in this case?
I found some stuff here and there, yet, I couldn't figure out my issue.
The graphed data were taken from this Ref[i].
From Wikipedia:

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded.

Other related questions: 1 and 2.

[i] Sheldahl, R.E. & Klimas, P.C., Aerodynamic characteristics of seven symmetrical
airfoil sections through 180-degree angle of attack for use in aerodynamic
analysis of vertical axis wind turbines , Albuquergue, NM: Sandia
National Laboratories, Report No.: SAND80-2114, 1980.

I'd be a little suspicious of that data... getting your best CL at around 45 degrees AoA???  That seems, um, improbable. What you probably need to look at is the spike around 10 degrees AoA.  Plotting what happens at AoA of 90 degrees or more seems unproductive, when you consider what that would look like in flight.

@RalphJ This is actually a common characteristics at extremely large AOA. It basically just follows a sine curve. But at 45deg L/D is 1:1 or less, so it's not very useful anyhow.

Are you asking about the definition, or how to computationally/experimentally find it?

@JZYL, I am asking how to find it, as shown in bold.

@AlFagera Then what are you trying to demonstrate with the two plots?

@JZYL, I need someone to help me find out what is the static stall angle for the given graph (even one of them is enough), didn't you get my point?

From your graph, it's around 12deg. What's there to find?

@JZYL: No, it's 9 degree. The peak is a bit before the middle of the first 20 degree box

@Peter Yes you're right

Being a symmetric airfoil, the NACA 0012 does not have a particularly high maximum lift coefficient, especially at low Reynolds numbers. And yes, 300,000 is considered a low value for the Reynolds number of airfoils. On the other hand, there is no other airfoil that has been tested so much. You will find data all over the place.
Generally, the angle of attack range of interest for aeronautics is where little to no separation of the flow along the surface occurs. This is restricted to a small range around 0° (when the flow is hitting the airfoil more or less head on). Larger separation reduces lift, so right before separation first lets lift drop a little, the stall angle of attack has been reached.

180° polar for several airfoils
But stall is not the end of lift. See above for 2D wind tunnel data collected by S. Hörner in his book Fluid Dynamic Lift. These results have been collected at a higher Reynolds number when the stall angle of attack is shifted to higher values. To be honest, a lot of factors influence this. In your case, even though there is another maximum at about 45°, the stall angle of attack is between 9° and 10°. Not more.


text:
Why do intermeshing helicopters use asymmetric airfoils?
I am referring to the Kaman K-max and the Kaman Huskie here. They both use NACA 23012, which is an asymmetric airfoil. Any reason as to why this is? Is there a different mechanic in play when intermeshing rotors are considered? Does Reynold's number change due to factors that don't affect normal helicopters?
Edit - I can see how my question can become very broad. My question would be answered if somebody could explain what factors come into play in intermeshing rotors that affect the choice of airfoil.

Pretty sure most helicopters use asymmetrical airfoils, not just the ones you mention, so I'd question if there is a valid premise underpinning this question.

Yeah, I edited the post.

Yep, I'd say the simple answer is, because they all do. The only flying machines that don't use them have special reasons not to. Intermeshing creates mechanical and control issues, but nothing related to the airfoil.

Helicopters may use either symmetrical or more commonly asymmetrical airfoils. It is not the symmetry that is important but the low pitching moment (Cm). The reason for using airfoils with low pitching moment is to avoid blade twist because unlike thick airplane wings helicopter blades are thin and relatively flexible. If you look at NACA 23012 you will see that the pitching moment stays around 0 at useful alpha angles (note the range between 5 to 15 degrees). As such this is not a bad airfoil for heli blades

Cambered airfoils started to appear on helicopters in the 80s, when as slebetman says, new composite materials made blades torsionally stiff enough to tolerate them.  This allowed higher Cls and a bit more efficiency from the blades.

Helicopters in general use asymmetric aerofoils because they are not required to fly inverted or produce negative lift. Intermeshing helicopters are no different in this regard.
The exception would be aerobatic model helicopters, which use symmetrical aerofoils because they are designed for similar performance either way up.


text:
Under FAA rules, can a private pilot act as PIC in aircraft furnished by someone else if they're not otherwise compensated?
Let's say I'm visiting a friend who happens to own an airplane, and I happen to be a private pilot holding a valid FAA PPL, as well as being familiar with the make and model my friend owns. Suppose my friend says "I'll show you around the avionics, and if you want to take her for a ride around the area, feel free. She's topped up on fuel, so you don't have to worry about it". Would it be legal for me to take them up on the offer if:

I were to fly alone, and not get compensated in any way, but also not bear any expenses related to the flight?

As above, but the friend also says "oh and by the way, I have a fuel card stashed in the cockpit, you can use it to refuel if you need to"

I were to take my wife and two friends, who are also visiting, as passengers?

As above, but again with fuel card

I were to act as PIC for a volunteer flight transporting sick patients, given that I receive no compensation for the flying, but my friend is furnishing the plane and the fuel onboard. They'd fly it themselves, but their IFR rating is not current and mine is

As above, but I also refuel the plane using their fuel card
As above, but the plane is provided by the organisation coordinating the flights, rather than my friend

I understand the basic limitations of the private pilot certificate, and that there's a broad prohibition on receiving any compensation for the carriage of passengers of cargo, including non-monetary compensation such as "building flying time". I am not, however, sufficiently versed in all the relevant interpretations to establish whether the above examples fall under the exemptions carved out for bona-fide common interest or volunteer flights.
Edit to clarify a point alluded to, but not explicitly addressed in one of the answers: my friend is not a business partner of mine, nor do I expect ever to do business with them. The intent of this (hypothetical) situation is that they are letting me fly their plane for fun, just as they could let me take their 700HP Dodge Charger Hellcat for a ride.
Edit 2: to make the unstated assumption explicit, I'm assuming that hours will be logged for those flights. The FAA has made it reasonably clear in the past that if hours are not logged, their compensatory aspect vanishes and thus renders many otherwise prohibited flights legal.

Related, but not a duplicate: https://aviation.stackexchange.com/questions/96239

There is a lot of bad info here. @DJClayworth has the best approach, although I don't know that he has the right answer. Yes, flight time could be considered compensation, but it is not at all always considered compensation. I periodically fly a friend's plane, to make sure it doesn't sit for extended periods of time. Is someone seriously making the claim that my time is so worthless that when I spend it to help a friend out he is compensating me? That my life, which was not at risk on the ground, is so worthless that a few minutes of flight is compensation?

@KennSebesta: Flight time is of value, and thus might be compensation. The FAA has made it abundantly clear, and specifically said that an ATPL rated pilot, with nothing they could be building time towards, is still receiving compensation when they get free flight time under their PPL privileges. Your situation is very close to #1 above, but whether it's for compensation or hire doesn't hinge on whether time is something of value, we already know it is. If anyone's common sense or intuition clashes with FAA's interpretation, then FAA's intepretation wins when legality is concerned. Sorry.

@mathrick to the contrary, read the FAA's Legal Interpretation: https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/Data/interps/1990/Lincoln-Soaring%20Society%20of%20America_1990_Legal_Interpretation.pdf. The FAA counsel specifically says, "It could be argued that the accumulation of flight time is not always of value to the pilot involved". Thus, "whether it's for compensation or hire doesn't hinge on whether time is something of value, we already know it is." is not correct, as from the FAA we know the opposite.

@KennSebesta: if you don't log time, then obviously you're not being compensated through building free time. Also, that counsel is no longer relevant, since 61.113(g) now specifically exempts glider towing.

@KennSebesta: also, the very next sentence in that counsel says: "The FAA does not consider it appropriate to enter into a case-by-case analysis to determine whether the logging of time is of value to a particular pilot, or what the pilot's motives or intentions are on each flight"

@mathrick Exactly! So to the discussion at hand, there is not a de facto assumption that flight time = value. With the FAA legal interpretation in mind, we might argue that there is insufficient clarity in the OQ. We can definitely make a lot of hay out of the fact that the FAA has wedged itself into the corner in such a way that it actively discourages pilots to log hours, but for the purposes of this question it is a very clear and shining example of how the FAA views "value": if you're doing it as a favor to a friend or associate, e.g. towing, then only the logging may have value.

@KennSebesta: OK, yes, I guess you got me there. It was an unstated assumption in my question that I want to log hours for those flights. It is implicitly there, in that I asked about the compensatory value of building time, but I didn't make it explicit. Will edit to reflect that.

@mathrick thanks. Unfortunately, I'm not sure anyone short of the FSDO can answer your question. It's bonkers that the act of flying can be illegal if I enjoy it but perfectly legal if I moan and groan about it. Maybe that's why we pilots are so curmudgeonly!

FAA:  "The FAA does not consider it appropriate to enter into a case-by-case analysis to determine whether the logging of time is of value to a particular pilot, or what the pilot's motives or intentions are on each flight."

Also FAA:  "We note that whether a private pilot is receiving something of value in exchange for acting as pilot in command is determined on a case-by-case basis and depends greatly on the purpose and objective of the flight"

The only real answer to this question is "it depends"...  On that basis I'm deleting my earlier attempt to answer this and punting.

For something to be "compensation," both sides must receive something of value. It's a "if you do X, I'll give you Y" kind of affair. "I'll give you Y unconditionally" is a "gift," not compensation. You are clearly receiving something of value in every case, so the question is if the owner is receiving something in return.
Cases 1 and 2 are essentially a short-term aircraft lease (albeit at sweetheart rates), and are definitely legal as long as the gift is truly unconditional. That said, you can expect the FAA and NTSB to be very skeptical of large unconditional gifts. Particular in case 2, if the two friends are also friends of the owner, it's easy to paint this as "the owner paid you to take their friends up for a flight."
Case 3 is definitely illegal. "If you act as PIC of this charity flight, I'll give you free flight time" is exactly the sort of quid pro quo that makes this illegal compensation instead of a gift.
All that said, this is all kind of a moot point. If the FAA decides to take action, you've basically already lost. While you might win the eventual NTSB appeal, you'll still have sunk tons of time and money into defending yourself.
If a bona fide friend is giving you a bona fide gift of flying their airplane, there's no reason the FAA should ever hear about it.

Let's go back to basics and read the actual law.

No person who holds a private pilot certificate may act as pilot in command of an aircraft that is carrying passengers or property for compensation or hire; nor may that person, for compensation or hire, act as pilot in command of an aircraft.

In legal terms "compensation" means that a person is doing something for someone else ("work" or "service" in legal terms), and in return receiving something of value - a payment. See this legal definition. It doesn't mean gifts. Real gifts are just gifts, and are not compensation, because there is no work or service involved.
When talking about compensation people love to quote this FAA opinion:

The specific quote is:

compensation is the receipt of anything of value. Reimbursement of
expenses [...] accumulation of flight time and goodwill in the form of
expected future economic benefit could be considered compensation.

This has led some people to suggest that any time as pilot is given something, such as free flight time or the loan of an aircraft, that automatically and always is considered compensation.
However the opinion doesn't say that. It says these things could be considered compensation - not they are or must be compensation.
One of the things deciding if flight time will be considered compensation is whether it satisfies the legal definition of compensation given above.  A gift doesn't satisfy it. It's safe to say the FAA knows that and that's one of the reasons they write "could be" in their opinion. The questions was about what forms of payment could be considered compensation, and they are answering with examples of things that could be compensation if they satisfy the legal definition.
As people have seen, the FAA takes a very wide view of what constitutes "a service", and what constitutes "compensation". Taking a passenger for a pleasure flight is a "service", and as we have seen receiving flying hours free or at a reduced rate can be a form of payment. But there have to be both the service and the payment for it to be considered "compensation". If that were not the case then gifting a pilot with an hour of flying time for his birthday would be illegal.
The FAA also takes a wide view of what a "service" is. Carrying passengers, even for a pleasure flight, is a service, and you cannot be compensated for it, even with reduced rate flights.  This is what leads to the strict rules on expense sharing. It should also be said that attempts to hide the service and recompense are illegal. If the friend "gifted" you flying time and you also "happened" to use it to take something of his somewhere, the courts would see through that as a pretence and rule it illegal.
Based on this, the friend allowing you to use his aircraft is a gift and not "compensation". You are not performing any service for him. The first situation is completely legal, as long as the gift is not conditional on you doing anything for him as a pilot. The second is also legal as long under the same conditions, provided that the people you take as passenger don't have a connection with the aircraft owner that makes this a transaction. Making a service "under the table" also doesn't change the legality, and you can't disguise a trade by saying "He gifted me the use of his plane and I then gifted him my services to fly him somewhere.". That's not going to believed by courts or the FAA.
The third situation is more complex. You are performing a service, but not for the aircraft owner. Fortunately this is a fairly common scenario and the FAA certainly has guidance on it.
It's worth noting that if the gifting of flight time without a service in return were illegal then that would make many things illegal that happen all the time. For example, most of my flight training was paid for by someone else - they gifted me that flight time. That commonly and openly occurs. But if all gifts are considered compensation then that would be illegal. It isn't illegal because I have performed no service that makes it "compensation".

Note that JimInTexas has the right answer also. I am writing this additional one because of the huge levels of misunderstanding around the law here.

Worth reading: https://aviation.stackexchange.com/a/96247/20394. The FAA specifically allows flying, so long as it's not logged. Ergo, flying for free is not de facto compensation.

In my opinion that would be providing a service and thus not technically legal if his friend pays costs.

That's an interesting take. I think you're right that I'm certainly providing a service, but it would be very hard to claim I'm getting just compensation, or anything approaching that. Either I'm working for free, or at such a reduced rate that the $10 in fuel burned is trivial compared to my costs in getting to the airport, getting the plane out, flying till the oil is warm, cleaning it off, and storing it.

It is worth noting that, on an incredibly strict interpretation everything is only done if the value to the person exceeds the cost. So in that case, it's impossible to do anything which isn't paid for in value. This is the same rationale as charging sales tax on all purchases, no matter how old or used the item is. And since the use tax is actually the rule of the land across the US, it is not trivial to dismiss that analogy. As a result, the reductio ad absurdum argument actually fails here, because in the homo economicus world no action is taken without greater compensation than cost.

@KennSebesta That's a (insanely) strict philosophical approach, and it's not true legally. The law recognizes that gifts do exist. It's also worth saying that all these interpretations are not binding. They are just the opinions of the FAA. Even if the FAA says "this is illegal" it's possible to go to court and argue that the FAA is wrong.

@DJClayworth I think we are in 100% agreement. I'm just pointing out that there is precedence for having this kind of insanely strict standard, and so that makes it harder to dismiss out of hand that "no one would ever enforce such a strict interpretation." So the FAA could theoretically go down that route, even though it has had the good sense not to.

All of your scenarios are legal under Part 91, except 3)3, which the FSDO might not like.
If own an airplane I can let my friends fly it.
Period.
Specifically:

Your friend lets you use his fuel card.  So what?  Unless there is an exchange of something of value between you and him nobody cares who pays for the gas.

Nobody cares if you take a friend, assuming you are current and are not flying in exchange for something of value from the owner, and you and your pax have a common purpose.

As far as sub points 1 and 2, that's clearly within part 91.  For insurance and FAA purposes I'd recommend conducting this flight through a Public Benefit non-profit, such as Angel Flight.

Subpoint #3:  If a FSDO guy is having a bad day he or she might want to quibble with this flight.   For sure if the 'organization' providing the airplane is a for profit entity you'd be out of line, because now your free flying would be considered compensation in exchange for helping the organization make a buck.
If the 'organization' providing is a non-profit, such a flying missionaries organization, then IMHO I don't think it'd be a problem, but you better coordination this flight with your FSDO and insurance company.
Note, when I say exchange I mean the pilot who lent you his plane gains some kind of economic value as a result of your flight.
What the FAA defiantly does not want to see is aircraft commercial operators 'hiring' private pilots to fly persons or cargo for free as part of a business operation.  That's the big NO-NO.
For example, about 30 years I knew a kid who had recently earned is private ticket at a little dirt strip in central Texas.  The local mechanic had deal with some Canadians to buy and refurb light planes, and to ferry these airplanes to airports near the Canada-US border.
The mechanic asked my friend if he could do any of these delivery flights. Of course my friend jumped at this chance to fly big cross countries for free.
The mechanic paid all the expenses for these flights, but didn't pay the pilots anything.
Under current FAA policy these flights would be 'go directly to jail, we are throwing away the key'!
Note to Mayor Pete and Captain Nolen:  This was over 30 years ago and the mechanic died years ago, as did my friend.

Regarding #1 & 2, that "something of value" is flight time.  Reference the FAA interpretations quoted in 757toga's answer.  "nobody cares who pays for the gas" um, the FAA does.  61.113(c) is pretty clear:  "A private pilot may not pay less than the pro rata share of the operating expenses of a flight with passengers, provided the expenses involve only fuel, oil, airport expenditures, or rental fees."

This is the correct answer. The flight time is not "compensation" because the pilot is not performing a service to be compensated for. There cannot be 'compensation" without a service provided.

@MichaelHall  61.113(c) is talking about sharing expenses with passengers. But the aircraft owner is not a passenger.

@DJClayworth, if the owner is not a certificated pilot, and they are on board the aircraft, they are a passenger.

@MichaelHall In the question the owner is not on board the aircraft.

@DJClayworth, ok, forget (c) then. Read (a).  It says nothing about the pilot providing a service.

Yes, because that's a basic assumption of the law. It is acting as PIC for hire or compensation that is forbidden. If the pilot is not receiving "compensation" in the legal sense then he is not breaking the law.